The present invention relates to an induction motor driving system, and more particularly to an induction motor driving system which can perform quick-response control of an induction motor at high speed and which can prevent the generation of heat by the induction motor.
The drive control of an induction motor wherein direct current is changed into alternating current by an inverter circuit and wherein the induction motor is driven by the alternating current, has been known as variable-frequency control (VF control) or variable-voltage variable-frequency control (VVVF control).
VF control is a method in which primary frequency, which is the output of the inverter circuit is varied in accordance with a command speed, while VVVF control is control in which the amplitude of the primary voltage is also varied in proportion to the variation of the primary frequency so as to control an output torque to a constant magnitude.
The conventional control systems as stated above deal with voltage and current to be applied to the induction motor, on the basis of the amplitude and frequency. Since they are control systems having a mean value fashion, they are incapable of fine control having quick-response. In order to ameliorate such a drawback, there has recently been developed and put into practical use a so-called "vector control system" which employs the pulse width control system and which controls the instantaneous value of the stator current of an induction motor, whereby the generation of a torque, truly equivalent to that of a shunt D.C. machine, can be effected. That is, the vector control system of the induction motor is based on the torque generating principle of the shunt D.C. machine and controls the instantaneous value of the stator current to perform the torque generation equivalent to that of the shunt D.C. machine.
In this manner, the vector control system makes instantaneous value control possible. However, as the rotational speed of the induction motor becomes high, the frequency of the primary current command becomes high. When using a transistor inverter, the high primary frequency approximates the chopping frequency of a transistor constituting the inverter, and the waveform of the primary input voltage approximates a rectangular wave, so that the current loop gain of the system declines. With the low current loop gain of the system, the current as commanded fails to flow through the induction motor.
Effective for eliminating such disadvantage is a method in which the vector control is shifted to the so-called slip control with a predetermined speed serving as the boundary. In this slip control, when the driving supply voltage of the induction motor is substantially constant, no problem occurs, but when it rises above a predetermined level, problems such as the generation of heat take place. It is therefore favorable to hold the driving supply voltage substantially constant. It is also desirable on the manufacturing side to manufacture the same driving devices for domestic needs and for foreign needs. In such case, however, the driving supply voltages differ widely. For example, the driving supply voltage is 200 V.+-.10% in Japan and is 230 V.+-.10% in U.S., so that it fluctuates from 180 V to 250 V. In other words, the supply voltage of 230 V.+-.10% is applied to the driving device designed for 200 V.+-.10%. In slip control, accordingly, the induction motor is overexcited (exciting current flows excessively) and generates heat, with the hazard of breakdown. Therefore, the quick-response of control of the induction motor and the safety of the equipment are not always compatible.